The chief objectives of the current project are (1) an epidemiological study of the incidence of chromosomal abnormalities in order to determine the etiology of nondisjunction; (2) a detailed study of the development and ultimate prognosis of newborns with sex chromosomal abnormalities; (3) search for the means to prevent or ameliorate the pathological course of these conditions; and (4) examination of the applicability of the current epidemiological study for use as a screening technique for genetic risk to human populations. We are continuing to monitor all births at two hospitals for aberrations of the sex chromosomes and for aberrations of chromosome 21. We are also following 53 children identified at birth as having abnormalities of one of the sex chromosomes with frequent physical, behavioral, perceptual, and intellectual examinations. They vary in age from one month to nine years. Thus far, they exhibit minor deviations from controls. We are also evaluating their family environments in order to identify environmental stresses to which they may be abnormally sensitive.